iOS动画 — 转场 — TabBar滑动

之前有涉及到 CATransition View 的转场，但实际上可能我们用到 VC 的转场可能更多一些。此时我先从一个需求出发举例，就是 ** tabBar 增加滑动屏幕切换 item 的效果**。

• 1、动画交互 —— UIViewControllerAnimatedTransitioning
• 2、什么时候需要动画交互 —— UITabBarControllerDelegate
• 3、真正执行的地方 —— UITabBarController

1、动画交互 —— UIViewControllerAnimatedTransitioning

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

typedef NS_ENUM(NSInteger,TabOperationDirection) {
    TabLeftDirection,
    TabRightDirection
};

@interface ScrollTabBarAnimator : NSObject <UIViewControllerAnimatedTransitioning>

@property (nonatomic, assign) TabOperationDirection tabScrollDirection;

@end

#import "ScrollTabBarAnimator.h"

@implementation ScrollTabBarAnimator

//动画持续时间
- (NSTimeInterval)transitionDuration:(id <UIViewControllerContextTransitioning>)transitionContext {
    return 0.3;
}

//动画执行效果
- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext {
    
    // 获取 toView fromView
    UIViewController* toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIViewController* fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIView *containerView = [transitionContext containerView];
    if (!toViewController || !fromViewController || !containerView) return;
    
    // 给 toView fromView 设定相应的值
    toViewController.view.transform = CGAffineTransformIdentity;
    fromViewController.view.transform = CGAffineTransformIdentity;
    CGFloat translation = containerView.frame.size.width;

    switch (self.tabScrollDirection) {
        case TabLeftDirection:
            translation = translation;
            break;
        case TabRightDirection:
            translation = -translation;
            break;
        default:
            break;
    }

    [containerView addSubview:toViewController.view];
    toViewController.view.transform = CGAffineTransformMakeTranslation(-translation, 0);
    // 真正的变化
    [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{
        fromViewController.view.transform = CGAffineTransformMakeTranslation(translation, 0);
        toViewController.view.transform = CGAffineTransformIdentity;
    } completion:^(BOOL finished) {
        fromViewController.view.transform = CGAffineTransformIdentity;
        toViewController.view.transform = CGAffineTransformIdentity;
        [transitionContext completeTransition:![transitionContext transitionWasCancelled]];
    }];
}

@end

2、什么时候需要动画交互 —— UITabBarControllerDelegate

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

@interface ScrollTabBarDelegate : NSObject <UITabBarControllerDelegate>

@property (nonatomic, assign) BOOL interactive;
@property (nonatomic, strong) UIPercentDrivenInteractiveTransition *interactionController;

@end

#import "ScrollTabBarDelegate.h"
#import "ScrollTabBarAnimator.h"


@interface ScrollTabBarDelegate ()

@property (nonatomic, strong) ScrollTabBarAnimator *tabBarAnimator;

@end

@implementation ScrollTabBarDelegate

- (instancetype)init {
    if (self = [super init]) {
        _interactive = NO;
        _interactionController = [[UIPercentDrivenInteractiveTransition alloc] init];
        _tabBarAnimator = [[ScrollTabBarAnimator alloc] init];

    }
    return self;
}

- (nullable id <UIViewControllerInteractiveTransitioning>)tabBarController:(UITabBarController *)tabBarController
                               interactionControllerForAnimationController: (id <UIViewControllerAnimatedTransitioning>)animationController {
    return self.interactive ? self.interactionController : nil;
}

- (nullable id <UIViewControllerAnimatedTransitioning>)tabBarController:(UITabBarController *)tabBarController
                     animationControllerForTransitionFromViewController:(UIViewController *)fromVC
                                                       toViewController:(UIViewController *)toVC {
    
    NSInteger fromIndex = [tabBarController.viewControllers indexOfObject:fromVC];
    NSInteger toIndex = [tabBarController.viewControllers indexOfObject:toVC];
    self.tabBarAnimator.tabScrollDirection = (toIndex < fromIndex) ? TabLeftDirection: TabRightDirection;
    return self.tabBarAnimator;

}

@end

3、真正执行的地方 —— UITabBarController

#import "ScrollTabBarController.h"
#import "ScrollTabBarDelegate.h"

@interface ScrollTabBarController ()

@property (nonatomic, strong) UIPanGestureRecognizer *panGesture;
@property (nonatomic, assign) NSInteger subViewControllerCount;
@property (nonatomic, strong) ScrollTabBarDelegate *tabBarDelegate;

@end

@implementation ScrollTabBarController

- (void)viewDidLoad {
    [super viewDidLoad];
    // 正确的给予 count
    self.subViewControllerCount = self.viewControllers ? self.viewControllers.count : 0;
    // 代理
    self.tabBarDelegate = [[ScrollTabBarDelegate alloc] init];
    self.delegate = self.tabBarDelegate;
    // 增加滑动手势
    self.panGesture = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(panHandle:)];
    [self.view addGestureRecognizer:self.panGesture];

}

- (void)panHandle:(UIPanGestureRecognizer *)panGesture {
    // 获取滑动点
    CGFloat translationX = [panGesture translationInView:self.view].x;
    CGFloat progress = fabs(translationX)/self.view.frame.size.width;
    
    switch (panGesture.state) {
        case UIGestureRecognizerStateBegan:
        {
            self.tabBarDelegate.interactive = YES;
            CGFloat velocityX = [panGesture velocityInView:self.view].x;
            if (velocityX < 0) {
                if (self.selectedIndex < self.subViewControllerCount - 1) {
                    self.selectedIndex += 1;
                }
            }
            else {
                if (self.selectedIndex > 0) {
                    self.selectedIndex -= 1;
                }
            }
        }
            break;
        case UIGestureRecognizerStateChanged:
        {
            [self.tabBarDelegate.interactionController updateInteractiveTransition:progress];
        }
            
            break;
        case UIGestureRecognizerStateEnded:
        case UIGestureRecognizerStateFailed:
        case UIGestureRecognizerStateCancelled:
        {
            if (progress > 0.3) {
                self.tabBarDelegate.interactionController.completionSpeed = 0.99;
                [self.tabBarDelegate.interactionController finishInteractiveTransition];
            }else{
                //转场取消后，UITabBarController 自动恢复了 selectedIndex 的值，不需要我们手动恢复。
                self.tabBarDelegate.interactionController.completionSpeed = 0.99;
                [self.tabBarDelegate.interactionController cancelInteractiveTransition];
            }
            self.tabBarDelegate.interactive = NO;
        }
            break;
        default:
            break;
    }
}

@end

大致效果就出来，还有点瑕疵，大致对自定义转场的感受分为：

• 第一步知道我们要什么效果，切换中应该发生什么？。

//返回动画的时间
- (NSTimeInterval)transitionDuration:(id <UIViewControllerContextTransitioning>)transitionContext;
//切换时的UIView的设置和动画
- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext; 

• 第二步，这个效果在什么地方实现。

//UINavigationController 的 delegate 属性遵守该协议。
<UINavigationControllerDelegate> 
//UITabBarController 的 delegate 属性遵守该协议。
<UITabBarControllerDelegate> 
//UIViewController 的 transitioningDelegate 属性遵守该协议。
<UIViewControllerTransitioningDelegate> 

上述例子用的就是UITabBarControllerDelegate，但实际上我们很多时候用的是UIViewControllerTransitioningDelegate。

• 第三步就是真正的实现，是否需要使用自定义的切换效果。

@protocol UIViewControllerTransitioningDelegate;

@interface UIViewController(UIViewControllerTransitioning)

@property (nullable, nonatomic, weak) id <UIViewControllerTransitioningDelegate> transitioningDelegate NS_AVAILABLE_IOS(7_0);

@end

detailVC.transitioningDelegate = self;
[self.navigationController presentViewController:detailVC animated:YES completion:nil];

至于上面那个例子，增加手势滑动，是特殊一点的场景，实际上还是 transitioningDelegate 的另一种展示。

总的说来，具体的效果还是得看第一步中的实现，目前我还没有深入，继续学习吧，决定多看看这个经典的VCTransitionsLibrary中的例子!

PS : 上述代码已放在：ScrollTabBar，欢迎提出问题。

备注参考：
http://kittenyang.com/uiviewcontrollertransitioning/
https://github.com/seedante/iOS-Note/wiki/ViewController-Transition